Nearly all of medical X-ray images used for diagnosis are images obtained by absorption contrast method. In absorption contrast method, contrast is created by attenuation differences in X-ray intensity when X-rays pass through a subject. On the other hand, there is suggested phase contrast method in which contrast is created by phase differences in X-rays and not by absorption of X-rays. For example, phase contrast imaging by which highly visible X-ray image can be obtained by emphasizing edges using X-ray refractions at the time of magnification image capturing is being carried out (for example, see patent documents 1 and 2).
Absorption contrast method is effective in image capturing of subjects which exhibit great X-ray absorption, such as bones. In contrast, phase contrast method can form images of tissues such as breasts, articular cartilages and soft tissues around articulars which are difficult to be captured as images by absorption contrast method because differences in X-ray absorption is small, and is expected to be utilized in X-ray image diagnosis.
As one type of phase contrast imaging, Talbot interferometer using Talbot effect has been considered (for example, patent documents 3 to 5). Tabot effect is a phenomenon in which when coherent light passes through the first grating having slits in constant cycles, grating images are repeated at constant intervals in the traveling direction of the light. These grating images are called self images. In Talbot interferometer, the second grating is disposed at the position where a self image is formed and measures the interference fringes (moire) that occur due to slightly displacing the second grating. Because moire is disturbed when an object is placed in front of the second grating, when X-ray image capturing is to be carried out by using Talbot interferometer, reconstruction image of a subject can be obtained by placing the subject in front of the first grating and emitting coherent X-rays onto the subject, and then, by performing arithmetic calculation of the obtained moire image.
Further, there is suggested a Talbot-Lau interferometer in which a multi-slit is disposed between the X-ray source and the first grating to increase the amount of X-ray emission (for example, see patent document 6). In a conventional Talbot-Lau interferometer, a plurality of moire images are captured in constant cycles while moving the first grating or the second grating (while relatively displacing both gratings). The multi slit is provided in order to increase the amount of X-ray emission.
The inventors of the present invention found out that an image equivalent to reconstruction image obtained by the conventional method can also be obtained by moving the multi-slit with respect to the first grating and the second grating in the Talbot-Lau interferometer, and this technique has been filed as Japanese Patent Application No. 2009-214483 (PCT/JP2010/53978).
In the above Talbot apparatus and Talbot-Lau apparatus, it is known that interference fringes in each moire image used in generation of reconstruction image need to be sharp and that the number of interference fringes needs to be small in order to obtain a sharp reconstruction image (for example, see non-patent document 1 (page 15)).